1. Field of the Invention
The present invention relates to the field of semiconductor memories. More specifically, the present invention relates to an electrically erasable programmable read-only memory (EEPROM) having a floating gate wherein information is stored in the form of an electrical charge on the floating gate.
2. Brief Description of the Prior Art
Field effect type metal-oxide-semiconductor (MOS) devices having floating gates have been utilized as semiconductor memories in the prior art. One such device is shown in U.S. Pat. No. 3,500,142 (Kahng) which teaches the use of a tunneling phenomena through thin oxide layers to charge a floating gate. Another device shown in U.S. Pat. No. 3,660,819 (Frohman-Bentchkowsky) utilizes an avalanche injection phenomena for charging the floating gate. Further, the devices taught in U.S. Pat. Nos. 3,984,822 (Simko I), 4,099,196 (Simko II) and 4,119,995 (Simko III) disclose floating gate devices which include a second gate or control gate and utilize hot carrier injection of channel electrons to charge the floating gate.
In addition to being electrically programmable, some of these devices are also electrically erasable. Particularly, the Kahng and Simko patents teach such electrically erasable PROMs (EEPROMs). However, these prior art EEPROM devices require either two different voltages, sometimes even of opposite polarities, to be applied to the control gate during writing and erasing or, alternatively, require the use of two distinct control gates to distinguish between the writing and erasing operations.